The present invention relates generally to plasma generation and, more particularly, to the use of a pulsed dc, atmospheric pressure plasma in cooperation with a spectrometer for generating, analyzing and detecting light emission from species present in the plasma.
Field-portable monitoring technologies are required in the chemical industry, for national defense, and for environmental protection in order to obtain real-time data concerning chemical emissions in air, to identify the sources of chemicals, and to reduce or eliminate the emissions of toxic chemicals. Analytical equipment such as gas chromatography (GC), high-performance liquid chromatography (HPLC), mass spectrometry (MS), inductively coupled plasma atomic emission spectrometer (ICP-AES), ICPMS, and GC/MS have been used to analyze chemical emissions in the environment. However, most of the currently available equipment is large, expensive, and unsuited for real-time field use. Therefore, there is a strong need to develop miniature, field-portable analytical instruments.
By employing a separation technique such as capillary gas chromatography, it is possible to develop a compact separation instrument. A field-portable gas chromatography/mass spectrometer instrument was reported by M. P Sinha and G. Gutnikov, Anal. Chem. (1991) 63, 2012-2016. A microbore column 3 m in length and having a 50 xcexcm inner diameter was used for separation. Such columns allow the carrier gas flow rate to be reduced to 0.05 atmxc2x7cm3xc2x7minxe2x88x921 and significantly reduces the weight and power needs of the mass spectrometer.
Plasma-based emission spectrometry was first used as a GC detector by A. J. McCormack et al., Anal. Chem. 1965, 37, 1470-1476. A microwave-induced plasma (MIP) provided the energy for molecular fragmentation and excitation. For a review of more recent developments, see for example xe2x80x9cSpeciation analysis by gas chromatography with plasma source spectrometric detectionxe2x80x9d by Ryszard Lobinski and Freddy C. Adams, Spectrochimica Acta Part B 52 (1997) 1865-1903. See also: xe2x80x9cLow Power Inductively Coupled Plasma Source for Element-Selective Atomic Emission Detection in Gas Chromatographyxe2x80x9d by L. J. Jerrell et al., Applied Spectroscopy 53 (1999), 245-248.
In xe2x80x9cA Molecular Emission Detector on a Chip Employing a Direct Current Microplasma, by Jan C. T. Eijkel et al., Anal. Chem. 1999, 71, 2600-260, and in xe2x80x9cA dc Microplasma on a Chip Employed as an Optical Emission Detector for Gas Chromatographyxe2x80x9d by Jan C. T. Eijkel et al., Anal. Chem. 2000, 72, 2547-2552, a direct current (dc), atmospheric pressure microplasma detector on a glass chip having a plasma volume of 50 nL is described. Hexane was detected at a level of 800 ppb with a linear dynamic range. Emission from the species in the plasma was observed using optical fibers through the wall of the glass chip in a direction perpendicular to the direction of the plasma gas flow.
A low-power microwave plasma detector was recently developed by U. Engel et al., Anal. Chem. (2000) 72, 193-197. A plasma having a longitudinal extension of 2-3 cm was generated with a forward power of 10-40 W. Because the power consumption is low, it is possible to operate the detector with a semiconductor microwave source powered by a car battery. Because of the low operational power, the rotational temperature of the plasma was reported to be about 650 K, thus the plasma has very low tolerance for water-loaded aerosols. Additionally, gas pressure and sample vapor clouds had a strong effect on plasma characteristics and performance.
Direct current (See, e.g., R. S. Braman and A. Dynako, Anal. Chem. (1968) 40, 95-106 and R. H. Decker, Spectrochim. Acta (1980) 35B, 19-31), alternating current (See, e.g., R. B. Costanzo and E. F. Barry, Anal. Chem. (1988) 60, 826-829), and high-voltage pulsed (See, e.g., W. E. Wentworth et al., Chromatographia (1992) 34, 219-225) plasma detectors have been reported. These detectors are simple to build and consume less power than other plasma sources. In xe2x80x9cPulsed Discharge Helium Ionization Detectorxe2x80x9d by W. E. Wentworth et al., supra, the authors describe a pulsed discharge helium ionization detector for gas chromatography. A pulsed, dc discharge is caused to occur between opposing, spaced-apart platinum wire electrodes. The volume of the discharge is small and is used to generate ions which are detected. Although a quartz window was placed at the end of the detector such that emission spectra from the discharge could be observed with a monochromator simultaneously with the ionization detection, the authors state that the window was unessential and was used merely to visually observe the discharge for color and stability. Additionally, only the permanent gases, including methane, and some inorganic gases were analyzed. There is no mention of detecting the light emitted from the pulsed discharge for organic or other species detection.
Accordingly, it is an object of the present invention to provide an apparatus and method for generating a plasma and detecting species present in the plasma using light emission characteristic thereof.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the pulsed, atmospheric pressure plasma apparatus for generating and analyzing light emission characteristic of species in the plasma hereof includes: an electrically insulating hollow tube open at one end and having a wall; a grounded metallic electrode piercing the wall of the hollow tube; a second metallic electrode piercing the wall of the hollow tube in the vicinity of said grounded electrode; means for flowing a gas in which the species is entrapped through the hollow tube; a high voltage dc pulse generator in electrical contact with the second electrode for generating a pulsed plasma in the gas flowing between the grounded electrode and the second electrode; and an optical spectrometer for spectrally resolving and detecting the light emission exiting the open end of the hollow tube characteristic of the species entrapped in the gas.
In another aspect of the present invention in accordance with the objects and purposes thereof the atmospheric pressure plasma method for generating and analyzing light emission characteristic of species in the plasma hereof includes the steps of: flowing a gas containing the species through an insulating hollow tube open at one end; generating a pulsed plasma between a grounded electrode and a second electrode disposed in the electrically insulating hollow tube by applying a pulsed dc voltage to the second electrode, whereby the species are excited; and spectrally resolving and detecting light emission exiting the hollow tube from the open end thereof.
Benefits and advantages of the present low-power, pulsed plasma source for molecular emission spectrometry include: (1) low power consumption (between 0.0 2 W and 5 W); (2) good tolerance to organic vapors; (3) small size; (4) high sensitivity to organic vapors; and (5) field-portable, real-time detection capability.